In one type of imaging optical system using a sensor, an infrared detector is placed into a cooling housing. The cooling housing maintains the detector at the required cryogenic temperature for efficient operation of the detector. The housing has an aperture therethrough so that the infrared detector may view an external scene through the aperture. In most cases, there is an optics subsystem that images the external scene onto the detector. The optics subsystem may be a telescope that alters the magnification of the scene when it is imaged onto the detector. For many applications, the optics subsystem must be as compact as possible, while remaining consist with the required optical performance.
In addition to the scene, some of the structure of the imaging optical system is within the field of view of the detector. The perceived temperature of the viewed structure is highly significant to the quality of the image produced by the infrared detector, because infrared wavelengths are associated with heat. The viewed portion of the structure that is cooled, such as the aperture of the cooling housing, does not adversely affect the image produced by the infrared detector. However, the viewed portion that is not cooled, such as the support structure of the optics subsystem, affects the viewed image as a form of noise.
One solution to this potential problem is to extend the cooling housing to include at least some of the portions of the structure that are within the field of view of the infrared detector. This solution greatly increases the volume that must be cooled, and reduces the rate at which the housing and its contents may be cooled from room temperature. This cooldown rate is a significant consideration for many applications, where the detector is initially at room temperature and must be rapidly cooled to the required cryogenic service temperature of the detector before operations may commence. Another approach is to use an off-axis optics subsystem. In this case, the optical elements, such as lenses and/or mirrors, must be highly powered and non-rotationally symmetric, with the result that they are difficult and expensive to manufacture, align, and test, and also require heavy supporting structure. The size and mass of the imaging optical system are also increased.
There is a need for an improved approach to such an imaging optical system. The present invention fulfills this need, and further provides related advantages.